I am new to brewing and having a LOT of fun. A friend who gave me his old brewing stuff and in all that he gave me was 14 22oz bottles of a malt brew that has been in his basement for (I don’t know how many) years. I chilled a bottle, cracked it open (still carbonated) and tried it. It was less than great but not something I would drink. I would like to use it for an experiment and see if I could turn it into something special. Any ideas?
beer batter?
Beer bread
Beer brats
marinade
Not much you can do to it now in drinkability, being it’s in the bottle.
Is there a process to use it in a new brew?
Not really. It’s possible to save some yeast and maybe build up enough for another batch. But, I would cut your losses and cook with it. Or drink it.
Depending on what beer it is, if you really want to use it, you could take a cap off, drop in a few hop pellets, cap it again. This would allow you to try 14 different varieties of hops for flavor and aroma.
Homebrew will lose flavor over time. I found some old bottles in the back of a closet a while back that I had no idea what was in them. Did the same thing you did, chilled them and cracked one open. I still don’t know what they were. Had almost no taste so I assume whatever it was the flavor faded over time.
If they are drinkable why not just have at 'em and enjoy. If not I would dump them out and save the bomber bottles.
If you knew someone with the appropriate equipment you could extract the “essential oils”. 
